Radio-controlled timepieces receiving radio waves including time information to adjust its own time based on that time information have been finding various applications such as clocks, wristwatches, etc. Used for the radio-controlled timepieces are long-wavelength radio waves of 40-200 kHz, particularly two frequencies of 40 kHz and 60 kHz in Japan and frequencies of 100 kHz or less mainly overseas. Although antennas as long as more than several hundreds of meters are needed to receive these radio waves efficiently, such long antennas cannot be used in wristwatches, keyless entry systems, RFID systems, etc. Generally used are thus magnetic sensor-type antennas comprising coils wound around magnetic cores, thereby exhibiting the same functions as those of long antennas.
A wristwatch is mainly constituted by a housing, a movement (driver module) and its peripheral parts (dial, motor, battery, etc.), a non-metal (glass) cover, and a rear metal cover. When an antenna is contained in a wristwatch, it is conventionally disposed outside the housing in many cases. However, the recent trend of increasing design appeal and reducing size and weight has required an antenna to be disposed in a housing. FIG. 12 shows one example of wristwatches containing an antenna in a housing 21. A movement 22 and peripheral parts 26 such as a battery, a motor for moving a pointer, etc. are disposed in the housing 21, and an antenna 1 is placed in a space defined by the housing 21, the movement 22, the peripheral parts 26 and the rear cover 24. The antenna 1 is shown by a solid line to clearly indicate its position, though the antenna 1 is actually not seen in the front view of FIG. 12.
JP 2003-110341 A discloses a small antenna for a radio-controlled timepiece comprising a magnetic core constituted by an amorphous metal laminate, and a coil wound around it. JP 8-271659 A discloses a small antenna comprising a magnetic core made of ferrite and a coil wound around it. Design is generally important for wristwatches, and their housings are preferably made of metals to have high-quality and decorative appearance. However, when the small antenna described in JP 2003-110341 A or JP 8-271659 A is mounted in a wristwatch with a metal housing, the metal housing acts as a shield to electromagnetic waves, resulting in drastically reduced receiving sensitivity.
JP 2002-168978 A discloses an antenna comprising a conductive seal member between a metal case and an antenna to keep a Q value. However, because the seal member is indispensable, it suffers restrictions in size reduction and design.
Japanese Patent 3,512,782 discloses an antenna comprising a main magnetic path comprising a coil wound around a magnetic core, and a magnetic sub-path comprising a magnetic core without a coil, an air gap being provided in part of a closed magnetic loop constituted by both magnetic cores, such that magnetic fluxes generated during resonance are less likely to leak outside. The antenna of Japanese Patent 3,512,782 selectively guides magnetic fluxes during resonance to the magnetic sub-path, thereby making the magnetic fluxes less likely to leak outside to suppress the reduction of a Q value due to an eddy current loss. However, the antenna of this structure suffers the problem that its S/N ratio is lowered even though the magnetic sub-path suppresses the reduction of the Q value. A lower S/N ratio leads to a higher error ratio of the received time information.
The keyless entry system enabling the remote operation of a key for a vehicle, etc. comprises a transmitting/receiving unit mounted to a vehicle equipped with an antenna for particular electromagnetic waves, and a transmitting/receiving unit (remote key) owned by a driver. In the keyless entry system, the vehicle-mounted transmitting/receiving unit periodically calls at a low frequency, and when a driver carrying a remote key with ID equal to the system enters into a transmittable area, the transmitting/receiving unit contained in the remote key returns encrypted ID in a UHF band to the vehicle to open the lock of the vehicle.
The radio frequency identification (RFID) system supplies and receives information stored in a tag via an antenna for predetermined electromagnetic waves. The RFID system comprises a transponder attached to an object, an interrogator, a computer, etc. The transponder, which is called “RFID tag,” comprises a memory for storing information, a wireless transmitting/receiving means for communicating with the interrogator wirelessly, and an antenna. The interrogator called “RFID reader/writer” transmits and receives an information-carrying carrier wave (radio wave or magnetic field) to and from the transponder to read the information of the transponder and write the information in the transponder. It comprises a controller for treating instructions from the computer, a wireless transmitting/receiving means for communicating with the transponder wirelessly, and an antenna. The information read by the interrogator is conveyed to a data-treating means such as a computer and used for managing the objects. The features of the RFID system are that because it reads and writes information without contact using radio waves (electromagnetic waves), dust and stain attached to the object are less likely to affect the reading and writing, that communication can be made even with obstacles except for metals, etc. between the interrogator and the transponder, and that simultaneous access is possible to pluralities of transponders in an RF field. The antenna of the present invention can be used as a receiving antenna in the transponder.
For instance, when an RFID tag, to which destination information, etc. are input, is mounted to a bus, etc., and when an RFID tag, to which timetable information is input, is embedded in a display board, etc. at a bus stop, various transportation information can be seen. Because such keyless entry systems and RFID systems comprise magnetic sensor-type antennas in metal housings or near metal parts, they also suffer the problem that the metal hinders the receiving of radio waves. Accordingly, the size reduction and sensitivity increase of an antenna are also required in these systems.